Isocyanates are widely used as production raw materials of such products as polyurethane foam, paints and adhesives. Although a plurality of reaction mechanisms can be considered for industrial production of isocyanates, the main industrial production method involves reaction of an amine with phosgene (phosgene method) as indicated in the following formula (i), and nearly the entire amount of isocyanates produced throughout the world are produced according to the phosgene method. However, the phosgene method has numerous problems.

Firstly, this method requires the use of a large amount of phosgene as raw material. Phosgene is extremely toxic and requires special handling precautions to prevent exposure of handlers thereof, and also requires special apparatuses to detoxify waste.
Secondly, since highly corrosive hydrogen chloride is produced in large amounts as a by-product of the phosgene method, in addition to requiring a process for detoxifying the hydrogen chloride, in many cases hydrolytic chlorine is contained in the isocyanates produced, which may have a detrimental effect on the weather resistance and heat resistance of polyurethane products in the case of using isocyanates produced using the phosgene method.
On the basis of this background, a method for producing isocyanate compounds is sought that does not use phosgene.
Although examples of such methods may include a method for synthesizing aliphatic isocyanate from an aliphatic nitro compound and carbon monoxide, and a method for converting an aliphatic amide compound to isocyanate by Hoffmann decomposition, both of these methods have poor yield and are inadequate for industrial application.
Methods for obtaining an isocyanate and a hydroxyl compound by thermal decomposition of N-substituted carbamic acid-O-alkyl ester compound have long been known, an example of which is the method of A. W. Hoffmann (see Non-Patent Document 1). This method enables a high yield to be achieved more easily than the methods described above, and the basic reaction employed in this method is indicated in the following formula (ii).

Thermal decomposition represented by the above general formula is reversible, and although the equilibrium thereof is biased towards the N-substituted carbamic acid-O-alkyl ester on the left side at low temperatures, the right side with the isocyanate and a alcohol side are advantageous at high temperatures. Thus, methods for obtaining isocyanate by thermal decomposition of N-substituted carbamic acid-O-alkyl ester are carried out at high temperatures (see, for example, Examples 12 and 13 of Patent Document 1). Here, although dependent on the ester group of the N-substituted carbamic acid-O-alkyl ester, the boiling point of N-substituted carbamic acid-O-methyl ester, for example, is 110° C. (during reduced pressure of about 2 kPa) (line 9 from the top of the right column of Non-Patent Document 2). On the other hand, the boiling point of hexamethylene diisocyanate, which is formed in the corresponding thermal decomposition reaction, is 130 to 140° C. (during reduced pressure of about 2 kPa) (Non-Patent Document 3). Namely, this indicates that the N-substituted carbamic acid-O-methyl ester has a lower boiling point than the product in the form of hexamethylene diisocyanate. Although the details of the thermal decomposition temperature of the N-substituted carbamic acid-O-methyl ester are not described, thermal decomposition proceeds at 200° C. or higher. In the case of carrying out the thermal decomposition reaction at the described temperature of 250° C. under reduced pressure, since these conditions exceed the boiling point of the N-substituted carbamic acid-O-methyl ester, the thermal decomposition reaction occurs in the gaseous phase. Since the raw material in the form of the N-substituted carbamic acid-O-methyl ester along with the product in the form of hexamethylene diisocyanate as well as the by-product in the form of methanol are also present in the gaseous phase, not only is it difficult to control the reaction, but various irreversible side reactions also occur. As indicated in the aforementioned publication by H. Schiff (Non-Patent Document 4) and the research by E. Dyer and G. C. Wright (Non-Patent Document 5), examples of such side reactions result in the formation of substituted ureas, biurets, urethodiones, carbodiimides and isocyanurates. In the case of gaseous phase thermal decomposition of N-substituted carbamic acid-O-alkyl esters, since the concentrations of both the isocyanate and N-substituted carbamic acid-O-alkyl ester are high in the gaseous phase, allophanate compounds form easily as indicated by the following formula (iii). The boiling points of these allophanate compounds are high since they are formed by a crosslinking reaction, and these allophanate compounds liquefy within the reactor simultaneous to their formation. Moreover, even when in this liquefied state, crosslinking among the allophanates proceeds easily due to thermal decomposition of N-substituted carbamic acid-O-alkyl ester groups. Thus, the allophanates gradually solidify resulting in clogging of the reactor.

In the case of using an N-substituted carbamic acid-O-alkyl ester as a raw material of a thermal decomposition reaction, these side reactions not only cause decreases in yield and selectivity of the target isocyanate, but also induce the formation of polymers during the production of polyisocyanate in particular, and depending on the case, can cause a situation that makes long-term operation difficult, such as causing the reactor to be clogged by precipitation of polymeric solids.
In addition, research has long been conducted on methods for producing N-substituted carbamic acid-O-alkyl esters as described above. The method described in Patent Document 2 contains the production of aliphatic carbamic acid-O-alkyl ester without using phosgene. In the first stage of this method, an aliphatic carbamic acid-O-alkyl ester is produced from an aliphatic primary amine and urea by reacting N,N′-dialkyl urea and a hydroxy compound, after which the primary amine formed as a by-product is isolated, recovered and returned to the first stage. Since this method not only has a low yield of carbamic acid ester formed, but also requires equipment for recycling the primary amine, the process is extremely complex and is not satisfactory for industrial application.
Methods using urea or carbonic acid derivative (such as carbonic acid ester or carbamic acid ester) have been proposed as alternative methods for producing N-substituted carbamic acid-O-alkyl ester.
The method described in Patent Document 3 contains reacting a primary diamine and alcohol with urea or carbonic acid derivative in the presence of a catalyst followed by conversion to N-substituted carbamic acid-O-alkyl ester. In the method described in Patent Document 4, N-substituted carbamic acid-O-alkyl ester is produced after first producing bis-urea from aliphatic primary amine, urea and alcohol, while the method described in Patent Document 5 involves the partial reaction of urea and alcohol in a first step, followed by supplying diamine to produce N-substituted carbamic acid-O-alkyl ester in a second step. Patent Document 6 describes a method for obtaining N-substituted carbamic acid-O-alkyl ester by reacting a primary amine and non-N-substituted carbamic acid-O-alkyl ester in the presence of alcohol at a ratio of NH2 groups to carbamate to alcohol of 1:0.8 to 10.0:0.25 to 50, at a temperature of from 160 to 300° C. and in the presence or absence of a catalyst, and then removing the ammonia formed as necessary. The method described in Patent Document 7 aromatic diisocyanate and/or polyisocyanate are produced by going through the following two steps. In the first step, an aromatic primary amine and non-N-substituted carbamic acid-O-alkyl ester are reacted in the presence or absence of a catalyst and in the presence or absence of urea and alcohol to obtain N-aryl carbamic acid-O-alkyl ester while removing the ammonia formed as a by-product as necessary, while in the second step, the N-aryl carbamic acid-O-alkyl ester is subjected to thermal decomposition to obtain aromatic isocyanate. The method described in Patent Document 8 contains producing N-alkyl carbamic acid-O-alkyl ester after first producing bis-urea from an aliphatic primary polyamine, urea and alcohol. Patent Document 9 describes a method for producing aliphatic O-alkyl monourethane by reacting an aliphatic primary amine and urea with an aliphatic alcohol.
However, as has been previously described, these thermal decomposition reactions for producing isocyanates from N-substituted carbamic acid-O-alkyl esters require high temperatures, and cause the formation of polymeric compounds attributable to undesirable side reactions as indicated in, for example, the above-mentioned formula (iii).
The majority of undesirable side reactions occur easily at higher temperatures. In addition, isocyanates formed in thermal decomposition reactions tend to increase the longer the duration of contact with the unreacted N-substituted carbamic acid-O-alkyl ester and other reaction components (including those in which a portion of the carbamic acid esters have become isocyanate groups in the case the thermal decomposition reaction raw material is a poly(N-substituted carbamic acid-O-alkyl ester)). Various methods have been proposed for obtaining a favorable isocyanate yield by inhibiting the formation of products of undesirable side reactions during thermal decomposition of N-substituted carbamic acid-O-alkyl esters.
Among N-substituted carbamic acid-O-esters (N-substituted carbamic acid-O-esters refer to carbamic acid esters containing a carbamic acid group and an organic group, and in the explanation of the present invention, indicate N-substituted carbamic acid-O-aryl esters, in which the organic group is derived from an aromatic hydroxy group and/or N-substituted carbamic acid-O-alkyl esters in which the organic group is derived from an alcohol), N-substituted carbamic acid-O-aryl esters, in which the ester group that composes the N-substituted carbamic acid-O-ester is an aromatic group (namely, carbamic acid ester group derived from an aromatic hydroxy group) offer the advantage of allowing the setting of a lower temperature for the thermal decomposition reaction as compared with N-substituted carbamic acid-O-alkyl esters, in which the ester group is an alkyl group (see, for example, Patent Document 10). In other words, the above-mentioned undesirable side reaction products can be inhibited if it were possible to set the thermal decomposition temperature to a low temperature.
On the other hand, the production of such N-substituted carbamic acid-O-aryl esters is more difficult than the production of N-substituted carbamic acid-O-alkyl esters. This is caused by the reactivities of the alcohol and aromatic hydroxy compound used as raw materials of their respective esterification reactions. The first reason for this is that aromatic hydroxy compounds have lower nucleophilicity than alcohols. The second reason is that the esterification reaction proceeds with difficulty due to the weakly acidic nature of aromatic hydroxy compounds.
Patent Document 11 describes a method for producing aliphatic N-substituted carbamic acid-O-aryl ester without using phosgene. In this method, N-substituted carbamic acid-O-aryl ester is produced oxidatively using a precious metal catalyst from a primary amine, carbon monoxide and an aliphatic alcohol or aromatic hydroxy compound. However, this method has the problems of a complex procedure and considerable costs. These problems refer to the use of highly toxic carbon monoxide, and the need to recover the catalyst from the product in the form of N-substituted carbamic acid-O-aryl ester due to the use of an expensive precious metal catalyst. Patent Document 12 describes a method for producing N-substituted carbamic acid-O-aryl ester by reacting an N-alkyl-N,N′-dialkyl urea, an aromatic hydroxy compound and hydrogen chloride gas. However, this method also has a complicated procedure and requires considerable costs. Namely, this method uses corrosive hydrogen chloride gas, consumes an expensive and uncommon urea compound, and results in difficulty in recovering the N-substituted carbamic acid-O-aryl ester from a hydrochloride of N,N′-dialkylamine formed as a by-product. In the method described in Patent Document 13, N-substituted carbamic acid-O-aryl ester is produced by a one-stage reaction between urea, aromatic hydroxy compound and aliphatic primary amine. In the method described in Patent Document 14, urea and an aromatic hydroxy compound are reacted in a first step, and N-substituted carbamic acid-O-aryl ester is produced by reacting with primary amine in a subsequent second step.
Technologies have also been disclosed for improving the low nucleophilicity of aromatic hydroxy compounds and the difficulty in shifting the equilibrium thereof. Patent Document 15 and Patent Document 16 disclose methods for producing aliphatic O-aryl urethane from a single-stage reaction of urea and/or O-aryl carbamate (non-N-substituted carbamic acid-O-aryl ester), aromatic hydroxy compound and aliphatic primary amine. In these methods, considerable improvement is made with respect to the removal of ammonia, such as by using reactive distillation or a method for introducing a large amount of inert gas. Patent Document 17 discloses a method for continuously producing urethane while continuously supplying a primary polyamine, urea and/or non-N-substituted carbamic acid ester and organic hydroxy compound to a reaction column to form the corresponding urethane and continuously extracting ammonia formed within the reaction column from the reaction column. On the other hand, Patent Document 18 describes a method that does not use a lowly nucleophilic aromatic hydroxy compound as described above. This method allows a corresponding N-alkyl carbamic acid-O-aryl ester to be obtained at a yield of 90 to 95% by reacting a primary alkyl amine with a diaryl carbonate in the presence of a solvent such as benzene, dioxane or carbon tetrachloride. This method has the advantage of allowing the obtaining of N-substituted carbamic acid-O-aryl ester at a low temperature as well as high selectivity. However, it is currently difficult to apply this method industrially due to the high cost of the diaryl carbonate.
In the case of methods using safe and inexpensive raw materials in the form of urea and carbonic acid derivatives, it is necessary to use an excess of urea or carbonic acid derivative based on the amino group of the primary amine in order to improve the yield based on the comparatively expensive primary amine. However, this does not mean that these methods have successfully inhibited side reactions or improved selectivity with respect to the primary amine.
The method described in Patent Document 19 contains recovering non-N-substituted carbamic acid-O-aryl ester from a resulting reaction liquid and recycling for use as a raw material of the reaction when producing N-substituted carbamic acid-O-aryl ester by reacting an aliphatic primary polyamine, aromatic hydroxy compound, urea and/or non-N-substituted carbamic acid-O-aryl ester. Namely, this method serves to reduce the amount used of the urea and/or non-N-substituted carbamic acid-O-aryl ester. In this method, after obtaining an aromatic hydroxy compound and isocyanic acid by thermal decomposition of non-N-substituted carbamic acid-O-aryl ester contained in the reaction liquid, and distilling the aromatic hydroxy compound at a low temperature, the isocyanic acid is again reacted with the distilled aromatic hydroxy compound and recovered in the form of non-N-substituted carbamic acid-O-aryl ester. However, in addition to this method having a complex procedure, the recovery rate of the non-N-substituted carbamic acid-O-aryl ester is unsatisfactory.
In this manner, a method for producing N-substituted carbamic acid-O-aryl ester using safe urea or carbonic acid derivative that satisfies the amounts of urea or carbonic acid derivative and primary amine has yet to be disclosed. This is due to the low nucleophilicity of aromatic hydroxy compounds as previously described as well as the low cationicity of the carbonyl carbons of the urea and carbonic acid derivative.
Despite the above-mentioned problems, since it would be extremely industrially useful to obtain isocyanates without using phosgene, various methods have been proposed for improving methods for producing isocyanates using N-substituted carbamic acid-O-esters in addition to those described above. There are methods that are carried out in a gaseous phase at high temperatures as well as methods that are carried out in a liquid phase under comparatively low temperature conditions. As was previously described, however, since there are cases in which side reactions occur during the course of thermal decomposition of N-substituted carbamic acid-O-alkyl esters resulting in the formation of precipitates, polymeric substances and obstructions in the reactor and recovery apparatuses or the formation of substances adhered to the reactor walls, economic efficiency is poor in the case of producing isocyanates over an extended period of time. Thus, although, for example, the use of chemical methods such as the use of a special catalyst (see Patent Document 20 or Patent Document 21), or a catalyst in combination with an inert solvent (see Patent Document 22) have been proposed for improving yield during thermal decomposition of N-substituted carbamic acid-O-alkyl esters, problems encountered during the production of N-substituted carbamic acid-O-aryl esters have yet to be solved. For example, the method described in Patent Document 23 is a method for producing hexamethylene diisocyanate. This method contains thermal decomposition of hexamethylene dicarbamic acid-O-ethyl ester in the presence of a catalyst mixture containing methyl toluene sulfonate and diphenyl tin dichloride while using dibenzyl toluene as a solvent. However, production and isolation of the starting components as well as purification and arbitrary recovery of the solvent and catalyst mixture are not described in detail, and the economic efficiency of this method is extremely low.
In the method described in Patent Document 24, N-substituted carbamic acid-O-alkyl ester is easily decomposed to isocyanate and alcohol in a carbon-containing fluidized bed without using a catalyst. However, the yield of isocyanate obtained in the thermal decomposition reaction is from about 83.8 to 98.7%, and although reaction by-products and the like are not described, since N-substituted carbamic acid-O-alkyl ester is still subjected to thermal decomposition and the resulting compounds contain isocyanate and alcohol that are susceptible to the occurrence of reverse reactions in the same manner as in the prior art, the above-mentioned side reactions cannot be said to be inhibited. In the method described in Patent Document 25, a circulating method to produce an alicyclic diisocyanate by reacting an alicyclic primary diamine, urea and alcohol to obtain an alicyclic dicarbamic acid-O-alkyl ester, followed by subjecting the alicyclic dicarbamic acid-O-alkyl ester to thermal decomposition. This method succeeds at reducing the amounts of materials used by recovering unreacted alcohol, non-N-substituted carbamic acid-O-ester and dialkyl carbonates, and recirculating a portion of the reaction mixture of the thermal decomposition step along with by-products to the initial step. However, this method requires the alicyclic dicarbamic acid-O-alkyl ester to be distilled at a high temperature of about 230° C. to remove residue unable to be used in the production method. Since this high distillation temperature is within the temperature range at which carbamic acid-O-alkyl esters undergo thermal decomposition, isocyanate groups formed during distillation ends up reacting with the alicyclic dicarbamic acid-O-alkyl ester resulting in the possibility of the formation of solid polymers. Although it is described in the examples that yield is maintained over a long operating time, there is no description regarding the presence of accumulation of polymers or clogging of the apparatus due to the occurrence of side reactions.
In addition, the method described in Patent Document 26 involves partially removing worthless by-products prior to thermal decomposition of N-substituted carbamic acid-O-alkyl ester. In this method, however, since N-substituted carbamic acid-O-alkyl ester is also removed together with the partially removed by-products, the yield of isocyanate based on primary amine and carbonic acid derivative is ultimately decreased. In addition, polymeric compounds are formed as a result of by-products remaining in the reactor without being discharged from the reactor being heated, and since these compounds adhered to the reactor, continuous operation over a long period of time is difficult.
In reactions for obtaining N-substituted carbamic acid-O-(alkyl or aryl) esters by reacting urea and carbonic acid derivative (or carbamic acid derivative) with a primary amine and alcohol or aromatic hydroxy compound, urea and carbonic acid derivative (or carbamic acid derivative) are used in excess to improve the selectivity of the expensive primary amine.
A reaction formula of N-substituted carbamic acid-O-alkyl ester in the case of using a primary amine and urea as raw materials is shown in the following formula (iv). Although an adequate amount of urea is present based on the primary amine in the initial phase of the reaction, as the reaction enters the latter phase, the concentrations of both (primary amine and urea) decrease resulting in the N-substituted carbamic acid-O-alkyl ester being present at a high concentration. As was previously described, the cationicity of urea and carbonyl carbons of carbonic acid derivatives is low (due to accepting electron donation by NH2 groups and alkoxy groups) and the difference in reactivity between carbonyl carbons of the product in the form of N-substituted carbamic acid-O-alkyl ester and the primary amine is small. Thus, unless the amount of urea is present in excess based on the primary amine, the reaction proceeds as indicated by formula (v) during the latter phase of the reaction. Namely, the primary amine reacts with the product in the form of N-substituted carbamic acid-O-(alkyl or aryl) ester causing it to be denatured to a compound having undesirable N,N-di-substituted urea bonds. In the case of using polyamine, since each amino group reacts successively, various denatured forms are formed in addition to that shown in the following formula (v). In addition, reactions also occur such as that based on the following formula (vii) involving reaction with isocyanate formed according to the following formula (vi), and it can be easily presumed based on a knowledge of organic chemistry and reaction rates that as the N-substituted carbamic acid-O-(alkyl or aryl) ester accumulates and urea concentration decreases, the formation of these denaturation products increases dramatically. Polymerized high molecular weight substances are naturally additionally formed based on the principles of the formulas (v), (vi) and (vii). Since compounds having N,N-di-substituted urea bonds formed due to this denaturation have low levels of reactivity, re-addition of dissociated alcohol becomes difficult. Although such reactions occur at high temperatures, since the formation of isocyanates by thermal decomposition of N-substituted carbamic acid-O-(alkyl or aryl) ester also begins to occur at high temperatures, this results in the occurrence of a diverse range of side reactions.
(Initial Reaction)

(Latter Reaction: Reaction with N-Substituted Carbamic acid-O-(alkyl or aryl) Ester)

(Latter Reaction: Reaction with Isocyanate)
Isocyanate Formation

Since polymeric substances formed based on the above-mentioned reactions have extremely low solubility in solvents and the like, they frequently adhere and solidify in the reactor, thereby making such methods industrially unsatisfactory. In response to such problems, although methods have been examined (see Patent Document 4) for producing N-substituted carbamic acid-O-alkyl ester by producing bis-urea from primary amine, urea and alcohol and then reaction the bis-urea with alcohol as previously described, these methods are targeted at reaction with a highly nucleophilic alcohol and are not carried out to solve the problem in thermal decomposition of the N-substituted carbamic acid-O-alkyl ester, thereby preventing such methods from solving the problems that occur during thermal decomposition of N-substituted carbamic acid-O-alkyl ester as described above. In addition, although a method for producing bis-urea has been proposed involving the reaction of molten urea and amine in the absence of a solvent (see, for example, Non-Patent Document 6), due to the high melting temperature of urea (about 135° C.), there are many cases in which the reaction proceeds non-uniformly resulting in the occurrence of urea denaturation reactions and reactions resulting in the formation of compounds having N,N-di-substituted urea bonds, thereby preventing the above-mentioned problems from being solved.
In addition, a method for purifying such polymeric substances by crystallization has also been developed (Patent Document 27). In this method as well, it is difficult to selectively crystallize compounds having similar structures at high yield, while on the other hand, energy is expended for separating solids and liquids as well as recovery of the crystallization solvent.